A global positioning system (GPS) measures a three-dimensional, global position of a radio receiver, using distances between the radio receiver and a number of earth-orbiting satellite transmitters. The receiver, usually mounted to a vehicle such as a commercial passenger aircraft, receives signals from the satellite transmitters at one or several different frequencies, such as L1, L2, and L5. Each signal indicates both the position of its transmitter and its transmission time, enabling the receiver, equipped with its own clock, to determine signal transit times and to estimate the distances to the transmitters.
A processor coupled to the receiver uses at least four of these distances, known as pseudoranges, to approximate or estimate the position of the receiver and the associated vehicle. The accuracy of these estimates, or position solutions, depends on a number of factors, such as changing atmospheric conditions and performance of individual satellite transmitters. If measurements based on several frequencies are used, ionospheric delay can be estimated and compensated for so that the pseudorange measurements are more accurate.
In commercial aircraft navigation and guidance, global positioning systems have traditionally only been used for determining position of an aircraft during non-critical portions of a flight, that is, between takeoff and landing. However, in recent years, researchers have started extending global positioning systems for use during landings. These extended systems have taken the form of ground-augmented or differential global positioning systems which typically include two to four ground-based GPS receivers, a ground-based differential correction processor (DCP), and a correction-data transmitter, all located in the vicinity of an aircraft landing area.
In 1998, the Federal Aviation Administration (FAA) initiated a program for developing and deploying such a navigational system known as the GPS-based Local Area Augmentation System, or GPS-based LAAS. As a result of this program, the FAA released Specification, FAA-E-2937A (Apr. 17, 2002), which establishes the performance requirements for a Category I LAAS Ground Facility (LGF) in the LAAS. Under this specification, the LGF monitors the satellite constellation; and provides LAAS corrections, integrity data, and approach data to approaching aircraft.
The LAAS or GBAS is a differential global positioning system (DGPS). The DGPS includes GPS and at least one ground station. As stated above, the GPS uses a number of orbiting position transmitting satellite stations and a receiver on an aircraft to determine the position of the aircraft with respect to ground. With the received satellite information, the receiver can determine the horizontal position, speed, and altitude of the aircraft. By adding a ground station, the DGPS can correct errors in the signal or the data transmitted by a satellite. As a result, the DGPS can determine the position of an aircraft with a high degree of accuracy.
The ground-based GPS receivers, each with a known position, work as typical GPS receivers in determining respective sets of pseudorange measurements based on signals from at least four earth-orbiting satellite transmitters. These pseudorange measurements may be calculated based on carrier phase and/or code phase information from one or several signal frequencies, such as L1, L2, or L5. These measurements are fed to the ground-based DCP, which uses the measurements and the known positions of the ground receivers to determine differential correction data. A correction-data transmitter then transmits the differential correction data to aircraft approaching the landing area. These approaching aircraft use the differential correction data to correct the corresponding pseudorange measurements of the on-board GPS receivers, providing better position solutions than possible using their on-board GPS receivers alone.
The corrected position solutions are then compared to a reference landing path to determine course deviations necessary to ensure that the aircraft follows the reference landing path. The course deviations are either provided to an autopilot system, which guides the aircraft during automatic landings, or displayed to the pilot. For either an autopilot based system or a manually controlled aircraft to function within safety limits set by the FAA, the position estimates are required to stay within minimum accuracy limits known as vertical and lateral alert limits.
The LGF broadcasts 1-sigma pseudorange ground (sigma_pr_gnd) values to the aircraft, which are used to calculate the vertical and lateral protection limits (or levels). These protection limits are used to monitor the airplane's vertical and horizontal position integrity and are directly tied to the safety of the airplane. Failure to stay within the specified alert limits should result in the issuance of an alert, signaling a pilot to abort the landing and to restart the landing process.
If the broadcast sigma_pr_gnd values do not reflect actual errors (i.e., the sigma_pr_gnd values are too small) the pilot may not receive an alert. Therefore, it would be beneficial for the LGF to monitor the actual sigma_pr_gnd values to ensure that the broadcast sigma_pr_gnd value is equal to or larger than the actual sigma_pr_gnd value.